Electronically tunable cavity resonator



Dec. 13, 1955 I 5 WHEELER 2,727,230

ELECTRONICALLY TUNABLE CAVITY RESONATOR Filed Sept. 20, 1950 5 Sheets-Sheet 1 Fig.l.

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WITNESSES:

INVENTOR fi ffiw Myron s. Wheeler. 3- w 6 1955 M. 5. WHEELER ELECTRONICALLY TUNABLEI CAVITY RESONATOR 3 Sheets-Sheet 2 Filed Sept. 20, 1950 B Era No 6 2 0 E; 35:62 .250

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o 9.52:4 m w uc=2c 00222 INVENTOR Myron S. Wheeler. g jzfiw AT ORNEY Dec. 13, 1955 M. 5. WHEELER 2,727,230

ELECTRQNICALLY TUNABLE CAVITY RESONATOR Filed Sept. 20, 1950 3 Sheets-Sheet. 3

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Reactance Tube Al I 54 so To Antenna A -i Reactance Tube 8| 1 55 ,5 To Antenna BM I Reoctance Tube DI Generator I kse 52 To Antenna 0 Reoctance Tube CI j ['51 53 To Antenna 6 ft Sine Wave Generator At Scan Phase Shift Frequency Network B4 Phase Shift Network 04 1 Phase Shift NetwarkD4 Total Power Output Fig.8.

WITNESSES: INVENTOR Myron S. Wheeler.

BY I jmyWzffx %g gjgig%c United States Patent Ofi 2,727,230 Patented Dec. 13, 1955 ice ELECTRONICALLY TUNABLE CAVITY RESONATOR Application September 20, 1950, Serial No. 185,758 Claims. (Cl. 343--100) This invention relates to high frequency, electromagnetic systems and apparatus, and relates more particularly to the use of electronically tunable, cavity resonators for controlling the flow of energy in such systems and apparatus.

In radar systems, scanners are used to periodically scan or sweep a predetermined pattern in spacewith mechanical mechanism for rotating or tilting the antennas or for spinning them about their axes. Such a mechanism is not only complicated, but is unsuited for rapid scanning.

This invention provides an electronic scanning system in which several antennas, each having a different radia tion pattern are energized in succession.

In one embodiment of this inve tion, a reactancc tube having a resonant cavity is placed in cascade with each antenna. and the cavities are tuned in success on to resonance at the frequency of the system, by electron beams. When the band pass of a cavity is at the frequency of the system, very little loss will occur so that the antenna connected to it will be energized. When a cavity is detuned from the frequency of the system, most of the energy is cut-off from its associated antenna, and handled instead by another antenna which at that time is tuned to the frequency of the system.

In another embodiment of this invention, a plurality of wave guides are used to connect a microwave generator directly to a plurality of antennas to be energized in succession. A reactance having a resonant cavity, is placed upon a branch of each wave guide, and their resonant frequencies are varied by swinging their control grid voltages in succession. When a cavity is tuned to the frequency of the system, power from the generator is transmitted with little loss to its associated antenna, at which time the other cavities are detuned from the gem orator frequency so that substantial reflection takes place at their Ta, and power to their associated antennas is decreased below maximum.

A feature of this invention is that the energy supplied to each antenna need not be switched off and on, but can be varied gradually from maximum to minimum so that the scan is uniform. There may thus be power in several antennas at the same time so that the radiated beam can assume an intermediate position.

An object of this invention is to control the flow of energy in a high frequency, electromagnetic system by tuning and detuning resonant cavities connected in the system, with electron beams.

Another object of this invention scanning systems.

Another object of this invention is to simplify elecironically controlled scanners for radar systems.

Another object of this invention is to provide a radar :canner having several antennas each having a different lirectivity characteristic, and to energize the antennas in uccessiou by tuning resonant cavities connected thereto, n succession, to the frequency of the energy to be transnitted.

Still another object of this invention is to provide a is to improve radar radar scanner having several antennas each having a differentdirectivity characteristic, and to pass maximum energy in succession to each antenna while maintaining energy in other antennas so that the scan is uniform.

The invention will now be described with reference to the accompanying drawing, of which:

Fig. 1 is a longitudinal section through a spiral beam reactance tube which may be used. with this invention;

Fig. 2 is a longitudinal section through the tube of Fig. 1, along a plane perpendicular to the plane on which the section of Fig. l is taken;

Fig. 3 i a longi dinal Section through another beam reactance tube which may be used with this tion;

Fig. 4 is a longitudinal section through the tube of Fig. 3, along a plane perpendicular to the plane on which the section of Fig. 3 is taken;

Fig- 5 is a diagrammatic view illustrating one embodiment of this invention using spiral beam reactanco tubes such as are illustrated by Figs. 1 and 2 of the drawing;

Fig. 6 is a view looking at the face of the reflector of Fig- 5;

Fig. 7 is a diagrammatic view illustrating another embodimcnt of this invention in which spiral beam reactanoc tubes of the type illustrated by Figs. 3 and 4 of the drawing, may be used; and

Fig. 8 is a graph illustrating the power applied to the antennas of the systems of Figs. 5 and 7.

This invention preferably makes use of spiral beam rcactance tubes of the types illustrated by Figs. 1-.4 of the drawing, and which are disclosed in more detail in my copending application Serial No. 185,757 filed concurrently herewith. Such a tube has a cavity tunable by an electron beam projected hereacross, and which constitutes a resonator. The electron stream under grid control passes between surfaces forming therebetwcen a capacitor. As the current in the electron stream is varied, the capacity between the capacitor surfaces varies the capacitance and resonant frequency of the cavity. When the cavity is connected to a microwave generator, energy from the generator will induce an alternat ng the capacitor surfaces to Magnetic flux from an spiral invenreversal of polarity of the capacitor surfaces.

As the current in the electron beam is varied by varying voltages applied to the control grid of the tube, the cavity is made resonant at different frequencies. Thus it may be made resonant at the frequency of a microwave genorator connected to i r t m y be demoed from the frequency of the generator. When it is so detuned. energy is reflected back to the driving source.

Referring now to Figs. 1 and 2 of the drawing, the spiral beam reactance tube there illustrated will be described. It is of a type in which energy from a source is passed directly through its resonator cavity, and comprises a cylindrical, evacuated, metal housing 10 having a relatively large chamber 11 therein and which is sealed at one end by an end cap 12 of magnetizable metal such as iron, and which functions in cooperation with an external magnet 13 as a pole piece.

Walls of the chamber opposite the cap 12 have a slot 14 therein at the center of the chamber, and which opens into a rectangular cavity 15 which is the resonator cavity of the tube. An accelerator grid 16 within the chamber 11 and grounded to the housing, extends across the slot 14. An elongated cathode 17 extends within the chamber between the cap 12 and the grid 16, and has he lead-in wires 18 for its heater. A control grid 19 is arranged across the slot 14 and between same and the cathode. The wall 20 of the housing, opposite the slot 14, forms the anode of the tube.

The ribs 21 which extend inwardly from the housing within the cavity 15, form therebetween a constricted space 22 through which the electrons from the cathode under control of the control grid, pass. The opposed faces of the ribs form capacitor surfaces so that the electron beam passing therebetween alters the capacitance of the cavity.

The side walls of the resonator cavity are provided with the H-transformer slots 25 and 26 which connect with the wave guides 27 and 28 respectively, and which are connected to couplings 29 and 30 respectively, one for connection to an external wave guide 31 to be connected to a load such as an antenna, and the other for connection with an external wave guide 32 to be con nected to a microwave generator in a system such as illustrated by Fig. of the drawing. The window closures 33 are provided within the couplings for enabling the interior of the tube to be maintained evacuated.

Referring now to Figs. 3 and 4 of the drawing, the spiral beam tube there illustrated is generally similar to that described in connection with Figs. 1 and 2, and differs therefrom in that the energy to be modulated or otherwise controlled, instead of passing through the resonator cavity, has a single wave guide 34 for connection to a generator whereby energy from the generator enters the wave guide 34 and induces an alternating cur rent field in the cavity 15. The magnetic pole piece 35 having the narrow slot 36 therein, forms the anode of this tube.

Variation of the voltage applied to the control grid of the tube varies the capacitance of the cavity and its resonant frequency. The wave guide 34 of this tube may be connected through a suitable coupling to a branch of a wave guide connecting the microwave generator and one of the antennas of the system of Fig. 7 of the drawing.

Referring now to Figs. 5 and 6 of the drawing, a parabolic reflector 40 of conventional design, has the four directional antennas A, B, C and D which are arranged in a circular sequence as close to the center of the reflector as possible. These antennas are excited in overlapping succession for sweeping a desired uniform pattern. They may be used in pairs for scanning in two dimensions.

The reactance tubes A1, B1, C1 and D1 which preferably are of the type illustrated by Figs. 1 and 2 of the drawing, are connected through the wave guides 41 to the microwave generator 42, and are connected through the wave guides 43 to their associated antennas A, B, C and D respectively.

The center frequency controls A2, B2, C2 and D2 of conventional design may be used for tuning the wave guides 41 to the center frequency of the system by varying their effective wave lengths. Similar controls may be used for tuning the wave guides 43.

The control grid of the reactance tube A1 is connected through the amplifier A3 to the sine wave generator 44 which generates the scan frequency. The control grid of the tube B1 is connected through the amplifier B3 and the phase shift network B4 to the generator 44. The control grid of the tube C1 is connected through the amplifier C3 and the phase shift network C4 to the generator 44. The control grid of the tube D1 is connected through the amplifier D3 and the phase shift network D4 to the generator 44.

The phase shift networks are of conventional design and shift the phase of the voltage from the generator 44 so that the voltage applied to the control grid of the tube B1 is 90 degrees out-of-phase with the voltage applied to the control grid of the tube Al, so that the voltage applied to the control grid of the tube C3 is 180 degrees out of phase with the voltage applied to the control grid of the tube A1, and so that the voltage applied to the control grid to the frequency of the tube D4 is 270 degrees out-of-phase with the voltage applied to the control grid of the tube A1.

The peak of the sine wave voltage from the generator 44 is thus applied in succession to the control grids of the reactance tubes A1, B1, C1 and D1. The reactance tubes may be so designed and operated that the normal value of the electron beam current of each results in its cavity being resonant at the frequency of the microwave generator 42. The reactance of each tube is varied by variation of its control grid voltage, the deviation from center frequency being proportional to the electron beam current. Thus the cavity of each tube can be progressively detuned from the center frequency by an increase in its electron beam current caused by an increase in positive voltage applied to its control grid.

When the peak of the positive voltage from the generator 44 is applied to the control grid of a tube, its resonant cavity is detuned from the frequency of the system. When minimum positive voltage is applied to the control grid of a tube then its resonant cavity is tuned to the frequency of the system. The scan voltage generator 44 and the phase shifting networks provide voltages to the control grids of the tubes in overlapping succession in such a manner that the grid voltage is decreased to a minimum on the grid of each of the tubes while the grid voltage increases or continues to be applied to the control grids of the other tubes. Thus when minimum grid voltage is applied, for example, to the control grid of the tube A1, its electron beam current is normal so that its cavity is tuned to the frequency of the system. At this time higher grid voltages are applied to the control grids of the tubes B1, C1, and D1 so that their cavities are detuned from the frequency of the system. At this time the antenna A is supplied with maximum energy, the antenna C is supplied with no, or minimum energy, while the antennas B and D receive intermediate energy values as illustrated by the graph of Fig. 8 of the drawing.

The generator 44 continues to cycle, gradually reapplying voltage to the control grid of the tube A1 and applying voltage to the grids of the tubes C1 and D1 While decreasing the voltage applied to the grid of the tube B1, resulting in its cavity being tuned to the frequency of the system, and in the cavities of the tubes A1, C1 and D1 being detuned from the frequency of the system. This results in the antenna B receiving maximum energy, the antenna D receiving no, or minimum energy, and in the antennas A and C receiving intermediate energy values.

This action continues, the cavity of the tube C1 being next tuned to the frequency of the system so that its associated antenna C receives maximum energy, while the cavities of the tubes A1, B1 and D1 are detuned from the frequency of the system, so that the antenna A receives no, or minimum energy and the antennas B and D receive intermediate energy values.

The cavity of the tube D1 is next tuned to the frequency of the system so that its associated antenna D is supplied with maximum energy, while the cavities of the tubes A1, B1 and C1 are detuned from the frequency of the system so that the antenna B receives no, or minimum energy while the antennas A and C receive intermediate energy values.

The cycling described in the foregoing is repeated at the scanning rate, the cavities of the tubes A1, B1, C1 and D1 being tuned in succession at intervals degrees apart,

of the microwave generator.

The reactance tubes A1, B1, C1 and D1 may, of course, be supplied from a conventional source which is not illustrated, with filament and anode currents. The connections of the electrodes of the tubes would be as usual except that since their anodes are electrically as well as mechanically connected to their metal housings, the Bl terminal of the anode voltage supply would be grounded instead of its negative terminal.

Fig. 7 of the drawing illustrates a system in which spiral beam reactance tubes of the type of Figs. 3 and 4 of the drawing may be used. In this system instead of the reactance tubes being connected in series in the wave guides connecting the microwave generator and the antennas, the microwave generator is connected to the antennas by the Wave guides 50, S1, 52 and 53 which have the branche 54, 55, 56 and 57 respectively, upon which a." Iucezi the reactance tubes A1, B1, C1 and D1 respective in the operation of the system illustrated by Fig. 7, the cavities of the reactance tubes are tuned in succession to the frequency of the system as described in connectio. with Fig. 6. When the cavity of each tube is timer; to the frequency of the system, its impedance is matched to that of the system so that no reflection occurs, and maximum energy flows through its associated wave guide to its associated antenna. At this time the cavities of the other tubes will be detuned from the frequency of the system so that reflection will take place at their Tconnections and their antennas will be supplied with less Sun maximum energy as illustrated by Fig. 8 cf the drawing. A; the resonant frequency of a cavity of a tube is Chtutgt. by swinging its control grid voltage so that it is no longer tuned to the frequency of the system, increase reflection will take place at its T-connection, and maximum power will be shifted to another antenna, the cavity of the as sociated reactance tube of which, is tuned to the frcqucncy of the system.

The branches of the wave guides preferably should each have a quarter wave length or a multiple thereof, at the frequency of. the system. Center frequency controls for adjusting the effective lengths of the Wave guides, similar to those illustrated by Fig. 6, may be used.

I claim as my invention:

1. A high frequency, electromagnetic system comprising a source of microwave energy, a plurality of loads for said source, a plurality of waveguides connecting said loads to said source a reactance tube operatively connected to each of said waveguides, each of said tubes having a resonant cavity included therein, an anode on one side of said cavity, a cathode on the other side of said cavity, and a control grid between said anode and cathode, an means for applying voltage in succession to said grids for tuning said cavities in succession to resonance at the frequency of said source.

2. A system as claimed in claim 1 in which the loads are antennas having different directional characteristics.

3. An electromagnetic system comprising a plurality of antennas circuinferentially spaced about a common center point, a signal generator for said antennas, means including a plurality of variable reactance resonant cavities connecting said antennas to said generator, electricalli responsive means included in each of said cavities for varying the reactance thereof, and electrical means for periodically increasing and decreasing in succession the reactances of said cavities, said electrical means including a source of alternating-current signals, connections between. the source of alternating current and the electrically responsive means of said cavities, and phase-shift devices included in all but one of said connections whereby the peak output of said alternating-current source will be applied to the electrically responsive means of said cavities in accordance with a predetermined pattern which is a function of the degree of phase-shift in each of said phase shift devices.

4. An electromagnetic system comprising a plurality of electrical loads, a signal generator for said loads, means including a plurality of variable reactance resonant cavities connecting said loads to said generator, electrically reisive means included in each of said cavities for vary- .g the reactance thereof, and electrical means for periodically increasing and decreasing the reactance of said cavities, said electrical means including a source of alternating-current signals, connections between the source of alternating current and the electrically responsive means of said cavities, and phase shift devices included in all but one of said connections whereby the peak output of said alternating-current source will be applied to the electrically responsive means of said cavities in succession.

5. An electromagnetic system comprising a plurality of loads, a signal generator for said loads, means including a plurality of variable reactance resonant cavities connecting said loads to said generator, an electron emitting device included in each of said cavities for varying the renctance thereof, and means for periodically increasing and decreasing in succession the reactance of said cavities, said means including a source of alternating-current signals, connections between said source of alternating current and the electron emitting device of each of said cavities, and means for applying the peak output of said source of alternating current to said electron emitting devices in succession.

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